Filter design for access points

ABSTRACT

Receiving filter design that reduces out-of-channel interference for APs is disclosed. An AP includes a first radio and a second radio disposed in a body of the AP. The first radio transmits first signals in a frequency band while the second radio receives second signals in the same frequency band. The AP includes an interference mitigation controller that determines a receiving filter for the second radio to mitigate interference between the first radio and the second radio based on the second signals received by the second radio when the first radio transmits the first signals in the frequency band. The interference mitigation controller applies the receiving filter to signals received by the second radio during a time period that the first radio is transmitting signals in the frequency band while the second radio is receiving signals in the frequency band.

BACKGROUND

The present disclosure relates to filter design for access points (APs),and more specifically, to receiving filter design for APs that canreduce out-of-channel interference.

Many APs have two radios where one of the two radios operates in the 2.4GHz Wi-Fi frequency band while the other radio operates in the 5 GHzWi-Fi frequency band. Because the frequency separation between the 2.4GHz frequency band and the 5 GHz frequency band is large, the two radiosoperating in the two different frequency bands do not cause seriousinterference to each other. In other words, in these APs, the two radioscan transmit and/or receive signals simultaneously without causingserious interference to each other. However, if a first radio istransmitting signals while a second radio in the AP is receiving signalsin the same frequency band, the signals transmitted by the first radiomay cause interference at the second radio, which can negatively affectits performance.

SUMMARY

One embodiment of the present disclosure provides an AP. The AP includesa body. A first radio is disposed in the body and configured to transmitfirst signals in a frequency band. A second radio is disposed in thebody and configured to receive second signals in the frequency band. TheAP also includes a controller. The controller is configured to determinea receiving filter for the second radio to mitigate interference betweenthe first radio and the second radio based on the second signalsreceived by the second radio when the first radio transmits the firstsignals in the frequency band. The controller is also configured toapply the receiving filter to signals received by the second radioduring a time period that the first radio is transmitting signals in thefrequency band while the second radio is receiving signals in thefrequency band.

One embodiment of the present disclosure provides a computer programproduct that includes a computer-readable storage medium having computerreadable program code embodied therewith. The computer readable programcode determines, for an access point comprising a first radio and asecond radio, a receiving filter for the second radio to mitigateinterference between the first radio and the second radio based onsecond signals received by the second radio in a frequency band when thefirst radio transmits first signals in the frequency band. The firstradio is disposed in a body of the access point and configured totransmit the first signals, and the second radio is disposed in the bodyof the access point and configured to receive the second signals. Thecomputer readable program code applies the receiving filter to signalsreceived by the second radio during a time period that the first radiois transmitting signals in the frequency band while the second radio isreceiving signals in the frequency band.

One embodiment of the present disclosure provides a method. The methodincludes transmitting first signals in a frequency band by a first radiodisposed in a body of an access point and receiving second signals inthe frequency band by a second radio disposed in the body of the accesspoint. The method also includes determining a receiving filter for thesecond radio to mitigate interference between the first radio and thesecond radio based on the second signals received by the second radiowhen the first radio transmits the first signals in the frequency band.The method further includes applying the receiving filter to signalsreceived by the second radio during a time period that the first radiois transmitting signals in the frequency band while the second radio isreceiving signals in the frequency band.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates an AP, according to one embodiment herein.

FIG. 2 illustrates a method to determine a receiving filter for the AP,according to one embodiment herein.

FIG. 3 shows that a radio in the AP receives signals from an externalnetwork device, according to one embodiment herein.

FIG. 4 illustrates a method to determine a receiving filter for the AP,according to another embodiment herein.

FIG. 5 shows that a first radio in the AP transmits signals while asecond radio in the AP receives the signals transmitted from the firstradio, according to one embodiment herein.

FIG. 6 shows a flowchart to apply both a time domain receiving filterand a frequency domain receiving filter, according to one embodimentherein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

In United States, the 2.4 GHz frequency band has only 3 non-overlappingchannels that are mostly saturated. However, the 5 GHz frequency bandhas 25 non-overlapping channels that provide more available channels tousers. In order to utilize the ample available channels in the 5 GHzfrequency band, an AP can include two radios that can operate in thesame frequency band. For example, the two radios in the AP can bothoperate in the 5 GHz frequency band on two different channels. For thistype of AP, when one radio in the AP is transmitting signals on onechannel while the other radio in the AP is receiving signals on anotherchannel in the same frequency band, the transmitted signals on onechannel may cause high out-of-channel interference to the receivedsignals on the other channel. The out-of-channel interference cannegatively affect the performance of the receiving radio in the AP.

Although the two different channels in the same 5 GHz frequency band arenon-overlapping channels, the two channels can be close to each other infrequency, e.g. the first channel may be the channel-36 with a centerfrequency at 5180 MHz, and the second channel may be the channel-40 witha center frequency at 5200 MHz. Also, the two radios in the same AP maybe located proximate to each other. Thus, when one radio in the AP istransmitting signals on the first channel while the other radio in theAP is receiving signals on the second channel, the transmitted signalson the first channel may have some energy leak into the second channel,and cause high out-of-channel interference on the second channel. Theembodiments below describe techniques for mitigating this interference.

FIG. 1 illustrates an AP 100, according to one embodiment herein. InFIG. 1, the AP 100 includes a body 101 which includes a first radio 102and a second radio 103. The two radios 102 and 103 operate in the samefrequency band, e.g. the 5 GHz frequency band. The radio 102 transmitsand receives signals through multiple antennas 104 (e.g., two antennasas shown in FIG. 1), and the radio 103 transmits and receives signalsthrough multiple antennas 105 (e.g., two antennas as shown in FIG. 1).That is, the two radios 102 and 103 each may have a Multiple-InputMultiple-Output (MIMO) transceiver. As shown, antennas 104 and 105 aremounted to the body 101 of the AP 100.

The body 101 of the AP 100 also includes a processor 106 and a memory107. The processor 106 may be any computer processor capable ofperforming the functions described herein. Although memory 107 is shownas a single entity, memory 107 may include one or more memory deviceshaving blocks of memory associated with physical addresses, such asrandom access memory (RAM), read only memory (ROM), flash memory orother types of volatile and/or non-volatile memory.

The memory 107 includes an interference mitigation controller 108. Theinterference mitigation controller 108 determines a respective receivingfilter for each of the radio 102 and the radio 103 to mitigateinterference between the two radios when communicating on differentchannels in the same frequency band. In one embodiment, the interferencemitigation controller 108 can be software. In other embodiments, theinterference mitigation controller 108 can be hardware, firmware orcombinations of software and hardware. For example, the interferencemitigation controller 108 may include hardware components on theprocessor 106 (e.g., a network processor).

In one embodiment, when the radio 102 transmits signals on a firstchannel in the 5 GHz frequency band while simultaneously the radio 103receives signals on a second channel in the 5 GHz frequency band, thesignals transmitted from the radio 102 cause out-of-channel interferenceto the signals received at radio 103. In one example, the interferencemitigation controller 108 determines a receiving filter for the radio103 using the signals received by the radio 103 on the second channelwhen the radio 102 transmits signals on the first channel in the 5 GHzfrequency band. Similarly, in another example, the interferencemitigation controller 108 determines a receiving filter for the radio102 using the signals received by the radio 102 on the first channelwhen the radio 103 transmits signals on the second channel in the 5 GHzfrequency band. In one embodiment, the first channel is a servingchannel of the radio 102, i.e., a channel assigned to the radio 102 toserve client devices. Similarly, in one embodiment, the second channelis a serving channel of the radio 103.

In some situations, applying the receiving filter decreases the MIMOreceiving capacity, which is not desired. Thus, in one embodiment, theinterference mitigation controller 108 does not determine the receivingfilter when applying the receiving filter decreases the MIMO receivingcapacity. In one embodiment, the interference mitigation controller 108determines the receiving filter only when one radio is transmittingsignals in the frequency band while the other radio is receiving signalsin the same frequency band. In this embodiment, the interferencemitigation controller 108 applies the receiving filter to only signalsreceived by the receiving radio during a time period when one radio istransmitting signals in the frequency band while the other radio isreceiving signals in the same frequency band. For example, theinterference mitigation controller 108 determines the receiving filterfor the radio 103 and applies the receiving filter determined for theradio 103 to signals received by the radio 103 only when the radio 102is transmitting signals and the radio 103 is receiving signals in thesame frequency band. In one example, when the radio 102 and the radio103 are both receiving signals, the interference mitigation controller108 does not determine the receiving filter for either of the tworadios. In another example, when the radio 103 is receiving signals andthe radio 102 is idle or powered off, e.g., not transmitting orreceiving signals, the interference mitigation controller 108 does notdetermine the receiving filter for the radio 103.

FIG. 1 is only one embodiment of the AP 100. In other embodiments, thetwo radios 102 and 103 both operate in the 2.4 GHz frequency band oranother frequency band different from the 5 GHz frequency band and the2.4 GHz frequency band. That is, the interference mitigation controller108 may generate and apply a receiving filter for other frequency bandsbesides 5 GHz. In other embodiments, one of the two radios 102 and 103is an XOR radio that can dynamically switch between the 2.4 GHz and 5GHz frequency bands. In other embodiments, the AP 100 includes more thantwo radios. For example, the AP 100 may include a dedicated 5 GHz radio,a dedicated 2.4 GHz radio, and an XOR radio. In other embodiments, thetwo radios 102 and 103 transmit and receive signals through one antennaor more than two antennas.

FIG. 2 illustrates a method 200 to determine a receiving filter for theAP, according to one embodiment herein. At block 201, the interferencemitigation controller 108 detects current or potential out-of-channelinterference in a frequency band, e.g., the 5 GHz frequency band, causedby the radio 102 at the radio 103. Because the radio 103 and the radio102 are co-located in the same AP, the interference mitigationcontroller 108 can detect whether the radio 102 causes or will causeout-of-channel interference to the received signals of the radio 103.

For example, the interference mitigation controller 108 detects currentout-of-channel interference when the radio 103 is receiving signals andthe radio 102 is simultaneously transmitting signals in the samefrequency band. In another example, the interference mitigationcontroller 108 predicts future out-of-channel interference when theradio 103 is scheduled to receive signals in the same time period theradio 102 is scheduled to transmit signals in the same frequency band.

After detecting current or potential out-of-channel interference causedby the radio 102 at the radio 103, at block 202, the interferencemitigation controller 108 triggers a training process to determine areceiving filter for the radio 103. In response, the interferencemitigation controller 108 initiates the training process from block 203to block 207 to determine the receiving filter for the radio 103, asdescribed below.

At block 203, the interference mitigation controller 108 controls theradio 102 to transmit training signals on a first channel of thefrequency band. In one embodiment, the training signals include constantand/or nearly 100% high-duty cycle frames on the first channel for atraining time period.

When the radio 102 is transmitting training signals on the first channelduring the training period, the interference mitigation controller 108controls the radio 103 to receive signals on a second channel of thefrequency band.

Some of the energy from the transmitted training signals leaks into thesecond channel and causes out-of-channel interference on the secondchannel. Thus, as shown at block 204, the radio 103 receivesout-of-channel interference caused by the training signals on the secondchannel.

At block 205, during the training period, the radio 103 receives testingsignals from an external network device. These testing signals enablethe interference mitigation controller 108 to determine the negativeimpact or strength of the out-of-channel interference. That is, todetermine the impact transmitting signals on the second radio has on theability of the first radio to accurately receive data from an externalsource, the interference mitigation controller 108 instructs an externaldevice to transmit the testing signals at the same time the first radiotransmits the training signals.

FIG. 3 shows the radio 103 in the AP 100 receiving testing signals froman external network device 300 during the training period, according toone embodiment herein. In one embodiment, the network device 300 is aneighboring AP. The neighboring AP 300 transmits testing signals, e.g.,beacons, to the radio 103 on the second channel. In one embodiment, atblock 205, the radio 103 receives beacons from the neighboring AP 300with a known frequency, e.g., 10 beacons per second, on the secondchannel. The out-of-channel interference caused by the transmittedtraining signals from the radio 102 interferes with the ability of theradio 103 to receive the testing signals emitted by the neighboring AP300. Thus, at block 205, the radio 103 may not receive all the beaconsfrom the neighboring AP 300. In one embodiment, the interferencemitigation controller 108 counts the number of actually received beaconsin the training time period when the radio 102 is transmitting trainingsignals on the first channel. Also, the interference mitigationcontroller 108 may calculate the number of expected beacons in thetraining time period based on the known frequency of the beacons. Theinterference mitigation controller 108 compares the number of actuallyreceived beacons with the number of expected beacons to determine theimpact or strength of the out-of-channel interference caused by thetransmitted training signals from the radio 102. For example, if thenumber of actually received beacons is significantly less than thenumber of expected beacons, it indicates that the out-of-channelinterference caused by the radio 102 is strong.

In another embodiment, the neighboring AP 300 is not assigned totransmit signals on the second channel—i.e., the channel used by theradio 103. Instead, the neighboring AP 300 is assigned a differentchannel to transmit signals. That is, the neighboring AP 300 does notnormally transmit signals on the second channel. In this embodiment, theinterference mitigation controller 108 instructs the neighboring AP 300to intermittently switch its radio to the second channel and transmit asequence of known testing frames (e.g., with a known frequency) to theradio 103 on the second channel during the training period. Similarly asabove, at block 205, the radio 103 may not receive all the known testingframes from the neighboring AP 300 due to the out-of-channelinterference. The interference mitigation controller 108 compares thenumber of actually received frames with the number of expected frames inthe training time period to determine the impact or strength of theout-of-channel interference caused by the transmitted training signalsfrom the radio 102.

In another embodiment, the network device 300 is a client device, e.g.,a cell phone. In this embodiment, the radio 103 in the AP 100 receivessignals from the client device 300. In one embodiment, the controller108 uses the radio 103 to send requests to the client device 300 to sendknown uplink testing signals or frames to the radio 103 on the secondchannel. For example, the radio 103 sends one or more Request-to-Send(RTS) messages to the client device 300 and thus expects to receive oneor more Clear-to-Send (CTS) messages from the client device 300.Similarly as above, at block 205, the radio 103 may not receive all theCTS messages from the client device 300 due to the out-of-channelinterference. The interference mitigation controller 108 compares thenumber of actually received CTS messages with the number of expected CTSmessages in the training time period to determine the impact or strengthof the out-of-channel interference caused by the transmitted trainingsignals from the radio 102. In another example, the radio 103 sends oneor more Block Acknowledgement Request (BAR) messages to the clientdevice 300 and thus expects to receive one or more Block Acknowledgementresponse (BA) messages from the client device 300. In another example,the radio 103 sends one or more Ping messages to the client device 300and thus expects to receive one or more Ping reply messages from theclient device 300.

In one embodiment, at block 206, the interference mitigation controller108 derives a cost function F_(c) indicating the impact of theout-of-channel interference. In one embodiment, the cost function F_(c)includes a parameter indicating the impact or strength of theout-of-channel interference. For example, F_(c) can be the ratio betweenthe number of actually received beacons from the neighboring AP 300 andthe number of expected beacons in the training time period In thisexample, F_(c) can be expressed as Equation (1) below:

$\begin{matrix}{F_{c} = \frac{{number}\mspace{14mu}{of}\mspace{14mu}{beacons}\mspace{14mu}{received}}{{number}\mspace{14mu}{of}\mspace{14mu}{expected}\mspace{14mu}{beacons}}} & (1)\end{matrix}$

In other embodiments, the cost function includes other parametersindicating the impact or strength of the out-of-channel interference.For example, F_(c) may include noise floor or SNR detected at the radio103 when radio 102 is transmitting training signals on the first channelin the training time period. In another example, F_(c) may includenumber of received training frames transmitted from the radio 102 in thetraining time period. In another example, F_(c) may include anestimation of duty cycle of received training frames transmitted fromthe radio 102 in the training time period. In another example, mayinclude the ratio between cyclic redundancy check (CRC) errors of thereceived frames and energy detection (ED) events or the ratio betweenCRC errors and Start of Packet (SOP) detection events in the trainingtime period.

In one embodiment, the cost function F_(c) includes multiple parametersindicating the impact or strength of the out-of-channel interference.For example, the cost function F_(c) may include both noise floor andthe ratio between CRC errors and SOP detection events. The interferencemitigation controller 108 can derive other examples of the cost functionF_(c) including multiple parameters, as understood in the art.

In one embodiment, after the cost function F_(c) is derived, at block206, the interference mitigation controller 108 determines filtercoefficients of the receiving filter for the receiving the radio 103 bymaximizing or minimizing the cost function F_(c). In other words, theinterference mitigation controller 108 finds the optimal filtercoefficients that maximizes or minimizes the cost function F_(c).

In one embodiment, the interference mitigation controller 108 maximizesor minimizes the cost function F_(c) based on a gradient searchapproach. In this example, assume the receiving filter is a 1×Nr vectorW, where Nr is the number of antennas 105 that receive signals for theradio 103. Further, the received signals by Nr antennas 105 at the radio103 in the training time period can be represented by an Nr×1 vector R.The interference mitigation controller 108 applies vector W to vector Ras WR and calculates the value of the cost function F_(c) as F_(c) (WR).In other words, the cost function F_(c) is a function of W. At block206, the interference mitigation controller 108 uses the gradient searchapproach to find an optimal W that maximizes or minimizes the value ofthe cost function F_(c) (WR).

In one embodiment, assume that the cost function F_(c) (WR) is definedas the ratio between CRC errors of the received frames and ED events inthe training time period, as mentioned above. The optimal W for thiscost function is the W that minimizes the value of this cost functionF_(c) (WR), which mitigates the out-of-channel interference the most.

The gradient search approach is an iterative algorithm. In the firstiteration, an initial value of W is set. For example, the initial W canbe a 1×Nr vector W₀ that all the elements in the vector are 1, i.e.,W₀=[1, 1, . . . , 1].

With W₀, the interference mitigation controller 108 calculates thedifferentiation (gradient) of F_(c) (WR) as Equation (2) below:

$\begin{matrix}{{\Delta\left( W_{0} \right)} = \left. \frac{{dF}_{c}({WR})}{dW} \right|_{W = W_{0}}} & (2)\end{matrix}$where Δ(W₀) is the gradient of F_(c) (WR) when W=W₀, and d representsthe differentiation operator.

With Δ(W₀), the interference mitigation controller 108 calculates thevalue of W for the second iteration as Equation (3) below:W ₁ =W ₀−γΔ(W ₀)  (3)where γ is a step size, which can be a number, e.g., 0.1, and can bechanged in each iteration. After the first iteration, F_(c) (W₁R)<(W₀R).

Similarly, in the second iteration, the interference mitigationcontroller 108 calculates the differentiation Δ(W₁) of F_(c) (WR) basedon Equation (2) with W=W₁ and calculates W₂ for the third iterationbased on Equation (3). After the second iteration, F_(c) (W₂R)<F_(c)(W₁R).

After N iterations, the value of F_(c) (WR) eventually converges to aminimum value F_(c) (W_(N)R) and the value of W after N iterations,i.e., W_(N), is the optimal receiving filter that minimizes F_(c) (WR).

In other embodiments, at block 206, the interference mitigationcontroller 108 can utilize the gradient search approach to find anoptimal W that maximizes or minimizes the value of different costfunctions F_(c) (WR), as understood in the art.

At block 207, the interference mitigation controller 108 applies thedetermined receiving filter to the signals received by the receivingradio. In one embodiment, the interference mitigation controller 108applies the determined receiving filter to only signals received by thereceiving radio during a time period that the transmit radio (e.g., theradio 102) is transmitting signals in the frequency band while thereceiving radio is receiving signals in the same frequency band. In oneembodiment, the interference mitigation controller 108 disables the useof the receiving filter when there is no out-of-channel interferencebetween the two radios, i.e., when the two radios are not workingsimultaneously in the same frequency band.

In one embodiment, at block 207, the interference mitigation controller108 applies W_(N) to the received signals R′ at the radio 103 asEquation (4) below:R″=W _(N) R′  (4)where R′ is an Nr×1 vector representing the received signals captured atNr receiving antennas 105. In one embodiment, R′ includes twocomponents. One component is the data signals transmitted to the radio103, e.g., from client devices. Another component is the out-of-channelinterference from the radio 102 because when the radio 103 is receivingdata signals on the second channel, the radio 102 is simultaneouslytransmitting signals on the first channel, which causes out-of-channelinterference to the data signals received by the radio 103. R″ is acomplex number representing the filtered signals received by the radio103 including the data signals and the mitigated out-of-channelinterference.

FIG. 4 illustrates a method for determining a receiving filter for theAP, according to another embodiment herein. Similarly as in FIG. 2, inFIG. 4, at block 401, the interference mitigation controller 108 detectscurrent or potential out-of-channel interference in a frequency band,e.g., the 5 GHz frequency band, caused by the radio 102 at the radio103. After detecting current or potential out-of-channel interference atblock 402, the interference mitigation controller 108 triggers atraining process to determine a receiving filter for the radio 103. Inresponse, the interference mitigation controller 108 initiates thetraining process from block 403 to block 406 to determine the receivingfilter for the radio 103, as described below.

FIG. 5 shows that the radio 102 transmits training signals while theradio 103 receives the training signals in the same frequency band,according to one embodiment herein. As shown in FIG. 5, at block 403,the radio 102 transmits training signals via antennas 104, whereantennas 104 include antenna 0 to antenna Nt−1. At block 404, the radio103 receives the training signals via antennas 105, where antennas 105include Nr antennas from antenna 0 to antenna Nr−1. Nt and Nr can be thesame number or can be two different numbers.

At block 403, the interference mitigation controller 108 controls theradio 102 to transmit training signals in the frequency band. In oneembodiment, the interference mitigation controller 108 controls theradio 102 to transmit training signals on a first channel, which isdifferent from the second channel used by the radio 103. However, inanother embodiment, the interference mitigation controller 108 controlsthe radio 102 to transmit training signals on the second channel, whichis the same channel used by the radio 103 to receive wireless signals.At block 404, the radio 103 receives the training signals on the secondchannel.

In one embodiment, the radio 103 transmits a CTS-to-self frame to itselfto reserve the second channel for a time period. During the reservedtime period, other APs and client devices using the second channel inthe coverage area of AP 100 cannot transmit signals. During the reservedtime period, at block 403, the radio 102 transmits training signals onthe first channel. That is, in this embodiment, the radio 102 transmitstraining signals on a different channel than the one used by the radio103. Simultaneously, during the reserved time period, at block 404, theradio 103 receives the training signals on the second channel. Sinceother APs and client devices using the second channel cannot transmitsignals, the radio 103 only receives the training signals transmittedfrom the radio 102 that leak into the second channel during the reservedtime period. That is, the radio 103 only receives out-of-channelinterference caused by the training signals from the radio 102. Asmentioned above, the training signals can include constant and/or nearly100% high duty cycle frames on the first channel for the reserved timeperiod.

In one embodiment, upon receiving the transmitted training signals fromthe radio 102, i.e., the out-of-channel interference from the radio 102,the radio 103 captures the In-Phase Quadrature (IQ) samples of thebaseband received signals in time domain. The IQ samples of the receivedsignals in time domain can be expressed as Equation (5) below:r[n]=[r ₀(n),r ₁(n), . . . ,r _(Nr−1)(n)]  (5)where r[n] is a 1×Nr vector, n is the discrete time index of thesampling time, and r_(i)(n) is the IQ sample captured at antenna i ofantennas 105 at time index n (i=0, 1, . . . Nr−1). r_(i)(n) is a complexnumber representing the combination or summation of sampled receivedsignals at antenna i transmitted from all antennas 104 at time index n,as indicated by arrows in FIG. 5.

In one embodiment, at block 405, the interference mitigation controller108 calculates the covariance matrix of [n]:r^(H)[n]r[n], where Hrepresents conjugate transpose and r^(H)[n]r[n] is an Nr×Nr covariancematrix. The interference mitigation controller 108 calculates asummation of the covariance matrices of r[n] for multiple discrete timeindices during the reserved time period as Equation (6) below:C=Σ _(n) r ^(H)[n]r[n]  (6)where C represents the summation of the covariance matrices of r[n] formultiple or all discrete time indices n during the reserved time period.That is, C is an Nr×Nr matrix representing the summation of multiplecovariance matrices r^(H)[n]r[n] when n has different values indicatingdifferent time indices during the reserved time period.

In one embodiment, at block 405, the interference mitigation controller108 determines the receiving filter for the radio 103 by calculating thesingular vectors of C by applying Singular Value Decomposition (SVD) toC as Equation (7) below:C=UDV ^(H)  (7)where U is an Nr×Nr unitary matrix, D is an Nr×Nr diagonal matrixcomprising eigenvalues of C, V is an Nr×Nr unitary matrix comprisingeigenvectors of C and H represents conjugate transpose.

The diagonal matrix D can be expressed as Equation (8) below:

$\begin{matrix}{D = {\begin{matrix}d_{0} & \; & \; & \; \\\; & d_{1} & \; & \; \\\; & \; & \ddots & \; \\\; & \; & \; & d_{{Nr} - 1}\end{matrix}}} & (8)\end{matrix}$where the diagonal matrix elements from d₀ to d_(Nr−1) are eigenvaluesof C.

Matrix V can be expressed as Equation (9) below:V=|V ₀ V ₁ . . . V _(Nr−1)|  (9)where matrix elements from V₀ to V_(Nr−1) are eigenvectors of C, andeach eigenvector is an Nr×1 vector.

In one embodiment, at block 405, the interference mitigation controller108 selects the singular vector in V associated with the smallest(lowest value) eigenvalue in D as filter coefficients for the receivingfilter. For purpose of explanation, DV^(H) can be written as Equation(10) below:

$\begin{matrix}{{DV}^{H} = {{{\begin{matrix}d_{0} & \; & \; & \; \\\; & d_{1} & \; & \; \\\; & \; & \ddots & \; \\\; & \; & \; & d_{{Nr} - 1}\end{matrix}}{\begin{matrix}V_{0}^{H} \\V_{1}^{H} \\\vdots \\V_{{Nr} - 1}^{H}\end{matrix}}} = {\begin{matrix}\begin{matrix}\begin{matrix}{d_{0}V_{0}^{H}} \\{d_{1}V_{1}^{H}}\end{matrix} \\\vdots\end{matrix} \\{d_{{Nr} - 1}V_{{Nr} - 1}^{H}}\end{matrix}}}} & (10)\end{matrix}$

From the above Equation (10), eigenvector V₀ is associated witheigenvalue d₀, and similarly eigenvector V₁ is associated witheigenvalue d₁ and so on. In one embodiment, diagonal elements in D canbe sorted so that d₀≤d₁≤ . . . ≤d_(Nr−1).

Assuming eigenvalue d₀ is the smallest eigenvalue among eigenvalues fromd₀ to d_(Nr−1), the interference mitigation controller 108 selects V₀ asthe filter coefficients for the receiving filter. The Nr×1 vector V₀ canbe expressed as Equation (11) below:

$\begin{matrix}{V_{0} = {\begin{matrix}{V_{0}(0)} \\{V_{0}(1)} \\\vdots \\{V_{0}\left( {{Nr} - 1} \right)}\end{matrix}}} & (11)\end{matrix}$where V₀(0) is the receiving filter coefficient for received signals atantenna 0 of antennas 105, and similarly V₁(0) is the receiving filtercoefficient for received signals at antenna 1 of antennas 105 and so on.By selecting V₀ as the filter coefficients for the receiving filter,when the received signals include out-channel-interference, themultiplication of V₀ and the out-channel-interference is close to zero,thus, the out-of-channel interference is mitigated.

In another embodiment, the interference mitigation controller 108selects multiple singular vectors in V associated with multiple smallesteigenvalues in D as filter coefficients for the receiving filter. Forexample, assuming eigenvalues d₀ and d₁ are the two smallest eigenvaluesamong eigenvalues from d₀ to d_(Nr−1), the interference mitigationcontroller 108 selects V₀ and V₁ as two sets of the filter coefficientsfor the receiving filter. In one embodiment, the interference mitigationcontroller 108 applies the selected multiple eigenvectors (e.g., V₀ andV₁) to the received signals separately, and combines the separatelyfiltered received signals to form the final received signals fordemodulation and/or decoding purposes.

In one embodiment, the interference mitigation controller 108 selectsmultiple singular vectors in V associated with multiple smallesteigenvalues in D as filter coefficients for the receiving filter basedon a threshold. For example, the interference mitigation controller 108selects multiple singular vectors in V associated with multiple smallesteigenvalues in D when the multiple smallest eigenvalues are all smallerthan a specified value.

After the receiving filter is determined, at block 406, the interferencemitigation controller 108 applies the determined receiving filter to thesignals received by the receiving radio.

For example, assuming V₀ is selected as the filter coefficients for theradio 103, the interference mitigation controller 108 applies V₀ to timedomain IQ samples at each receiving antenna from antenna 0 to antennaNr−1 as Equation (12) below:r″[n′]=r′[n′]V ₀  (12)where r′[n′] is an 1×Nr vector representing the filtered IQ samplescaptured at receiving antennas 105 from antenna 0 to antenna Nr−1 attime index n′ (similarly as r[n] in Equation (1)). In one embodiment,r′[n′] includes two components. One component is the data signalstransmitted to the radio 103, e.g., from client devices. Anothercomponent is the out-of-channel interference from the radio 102. r″[n′]is a complex number representing the filtered received IQ sample at timeindex n′ received by the radio 103 including the data signals and themitigated out-of-channel interference.

The embodiment above discloses that the interference mitigationcontroller 108 determines filter coefficients for the receiving filterbased on time domain IQ samples, thus, the receiving filter is a timedomain filter and the filter coefficients of the time domain filter aredetermined for each receiving antenna.

In another embodiment, instead of the radio 102 in FIG. 5 transmittingtraining signals on a different channel than the one used by the radio103, at block 403, the radio 102 transmits training signals on thesecond channel during the reserved time period, and at block 404, theradio 103 receives the training signals on the second channel during thereserved time period. In this embodiment, because the radio 102transmits training signals on the same channel as the radio 103, theradio 103 can estimate the frequency domain channel coefficients betweenthe radio 102 and the radio 103. For example, the radio 102 can transmittraining symbols that are known to the radio 103 and the radio 103 canestimate the frequency domain channel coefficients based on the knowntraining symbols. Based on the estimated frequency domain channelcoefficients, the interference mitigation controller 108 can determinefrequency domain filter coefficients for the receiving filter. That is,the interference mitigation controller 108 can determine a frequencydomain filter.

In one embodiment, at block 403, the radio 102 transmits known trainingsymbols represented by a vector S through all the antennas 104 fromantenna 0 to antenna Nt−1. In one embodiment, S is an OrthogonalFrequency Division Multiplexing (OFDM) symbol. The OFDM symbol S istransmitted from the radio 102 to the radio 103 in time domain. At block404, the radio 103 receives the time domain OFDM symbol S and performsFast Fourier Transform (FFT) to transform S from time domain tofrequency domain for further processing, as understood in the art.

In frequency domain, S can be expressed as Equation (13) below:

$\begin{matrix}{S = {\begin{matrix}{S(0)} \\{S(1)} \\\vdots \\{S\left( {M - 1} \right)}\end{matrix}}} & (13)\end{matrix}$where S has M subcarriers (sub-channels of the second channel), and S(0)is the training symbol on subcarrier 0, and similarly S(1) is thetraining symbol on subcarrier 1 and so on.

Considering one subcarrier k (0≤k≤ . . . ≤M−1), in frequency domain, thereceived symbol vector R(k) at all receiving antennas on subcarrier kfor the radio 103 can be expressed as Equation (14) below:

$\begin{matrix}{{R(k)} = {{{H(k)}{X(k)}} = {{\begin{matrix}{H_{0,0}(k)} & {H_{0,1}(k)} & \ldots & {H_{0,{{Nt} - 1}}(k)} \\{H_{1,0}(k)} & {H_{1,1}(k)} & \ldots & {H_{1,{{Nt} - 1}}(k)} \\\vdots & \vdots & \ddots & \vdots \\{H_{{{Nr} - 1},0}(k)} & {H_{{{Nr} - 1},1}(k)} & \ldots & {H_{{{Nr} - 1},{{Nt} - 1}}(k)}\end{matrix}}{\begin{matrix}{S(k)} \\{S(k)} \\\vdots \\{S(k)}\end{matrix}}}}} & (14)\end{matrix}$where R(k) is an Nr×1 vector representing the frequency domain receivedsymbols at the radio 103 on subcarrier k on the second channel, H(k) isan Nr×Nt matrix representing the frequency domain channel between theradio 102 and the radio 103 on subcarrier k. For example, H_(0,0)(k) isthe complex channel coefficient between transmit antenna 0 of antennas104 and receiving antenna 0 of antennas 105 on subcarrier k, andsimilarly H_(Nr−1,Nt−1)(k) is the complex channel coefficient betweentransmit antenna Nt−1 of antennas 104 and receiving antenna Nr−1 ofantennas 105 on subcarrier k, and so on. X(k) is an Nt×1 vectorincluding the frequency domain training symbol S(k) transmitted bytransmit antennas from antenna 0 to antenna Nt−1 on subcarrier k.

Since S(k) is a known training symbol to the radio 103, based on thereceived symbol vector R(k), the interference mitigation controller 108can estimate H(k), as understood in the art.

Similarly as Equation (7), when H(k) is estimated, at block 405, theinterference mitigation controller 108 calculates the singular vectorsof the covariance matrix H(k)^(H)H(k) of H(k) by applying SVD toH(k)^(H)H(k) as Equation (15) below:H(k)^(H) H(k)= U (k) D (k) V (k)^(H)  (15)where Ū(k) is an Nr×Nr unitary matrix, D(k) is an Nr×Nr diagonal matrixcomprising eigenvalues of H(k)^(H)H(k), V(k) is an Nr×Nr unitary matrixcomprising eigenvectors of H(k)^(H)H(k) and H represents conjugatetranspose.

Similarly as explained above, at block 405, the interference mitigationcontroller 108 selects one or multiple singular vectors in V(k)associated with one or multiple smallest eigenvalues in D(k) as filtercoefficients of the receiving filter for the radio 103 on subcarrier k.In this way, the interference mitigation controller 108 can determinefilter coefficients of the receiving filter for the radio 103 on eachsubcarrier from subcarrier 0 to subcarrier M−1, respectively. In anotherembodiment, the interference mitigation controller 108 determines thereceiving filter for the radio 103 by calculating the singular vectorsof the channel matrix H(k), as understood in the art.

After the receiving filter is determined, at block 406, the interferencemitigation controller 108 applies the determined receiving filter foreach subcarrier to the signals received by the receiving radio.

For example, assuming V ₀(k) is a Nr×1 vector in V(k) and is selected asthe filter coefficients for the radio 103 on subcarrier k, theinterference mitigation controller 108 applies V ₀(k) to Nr×1 frequencydomain received symbol vector R′(k) on subcarrier k as Equation (16)below:R″(k)= V ₀(k)^(T) R′(k)  (16)where T represents transpose. In one embodiment, R′(k) includes twocomponents. One component is the frequency domain data symbolstransmitted to the radio 103, e.g., from client devices on subcarrier k.Another component is frequency domain out-of-channel interference fromthe radio 102 to the data symbols received by the radio 103 onsubcarrier k. R″(k) represents the filtered received signals infrequency domain including the data symbols and the mitigatedout-of-channel interference on subcarrier k. In this way, theinterference mitigation controller 108 applies the determined receivingfilter for each subcarrier to the signals received by the receivingradio.

The embodiment above discloses that the interference mitigationcontroller 108 determines filter coefficients for the receiving filterbased on frequency domain channel matrix, thus, the receiving filter isa frequency domain filter. Also, the filter coefficients of thefrequency domain filter are determined for each receiving antenna oneach subcarrier.

The embodiments described in FIG. 4 and FIG. 5 can be used separately orcan be combined. For example, the interference mitigation controller 108can first determine the time domain receiving filter to mitigate part ofthe out-of-channel interference at each receiving antenna in timedomain, and then the interference mitigation controller 108 candetermine the frequency domain receiving filter to further mitigate theout-of-channel interference on each subcarrier in frequency domain.

FIG. 6 shows a flowchart 600 to apply both the time domain receivingfilter and the frequency domain receiving filter, according to oneembodiment herein. At block 601, the interference mitigation controller108 determines the time domain receiving filter based on received timedomain IQ samples, as explained above. At block 602, the interferencemitigation controller 108 determines the frequency domain receivingfilter based on received frequency domain symbols, as explained above.At block 603, the interference mitigation controller 108 applies boththe time domain receiving filter and the frequency domain receivingfilter to mitigate the out-of-channel interference. In anotherembodiment, the interference mitigation controller 108 can firstdetermine the frequency domain receiving filter and then determined thetime domain receiving filter.

Although the above embodiments determine a receiving filter for theradio 103, the above embodiments can be similarly used to determine areceiving filter for the radio 102, when the radio 102 is receivingsignals while the radio 103 is transmitting signals in the samefrequency band, as understood in the art.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

In the preceding, reference is made to embodiments presented in thisdisclosure. However, the scope of the present disclosure is not limitedto specific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practicecontemplated embodiments. Furthermore, although embodiments disclosedherein may achieve advantages over other possible solutions or over theprior art, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the scope of the present disclosure. Thus,the following aspects, features, embodiments and advantages are merelyillustrative and are not considered elements or limitations of theappended claims except where explicitly recited in a claim(s). Likewise,reference to “the invention” shall not be construed as a generalizationof any inventive subject matter disclosed herein and shall not beconsidered to be an element or limitation of the appended claims exceptwhere explicitly recited in a claim(s).

Aspects of the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.”

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. An access point, comprising: a body; a firstradio disposed in the body and configured to transmit first signals in afrequency band that includes at least a first channel and a secondchannel different from the first channel; a second radio disposed in thebody and configured to receive second signals in the frequency band; acontroller configured to: train a receiving filter for the second radioto mitigate interference between the first radio and the second radio onthe second channel of the frequency band, wherein training the receivingfilter comprises: transmitting a request to transmitting devices, otherthan the first radio, to not transmit signals in the frequency bandduring a reserved time period, wherein the request comprises aClear-to-Send-to-self frame; transmitting a training signal, by thefirst radio on the first channel in the frequency band during thereserved time period, and adjusting the receiving filter to decreasereception of testing signals received by the second radio on the secondchannel during the reserved time period in which the first radio istransmitting the training signal, wherein the testing signals areportions of the training signal that leak into the second channel duringthe reserved time period; apply the receiving filter to signals receivedby the second radio in response to determining that the first radio iscurrently transmitting the first signals while the second radio isreceiving the second signals; and remove the receiving filter fromsignals received by the second radio in response to determining that thefirst radio is not currently transmitting signals.
 2. The access pointof claim 1, wherein the frequency band comprises a 5 GHz Wi-Fi frequencyband.
 3. The access point of claim 1, wherein the first radio isconfigured to transmit the first signals in the frequency band through afirst plurality of antennas disposed on the body, and wherein the secondradio is configured to receive the second signals in the frequency bandthrough a second plurality of antennas disposed on the body.
 4. Theaccess point of claim 1, wherein the first radio is configured totransmit the first signals on the first channel in the frequency bandand wherein the second radio is configured to receive the second signalson the second channel in the frequency band.
 5. The access point ofclaim 1, wherein the controller is further configured to: determine atime domain receiving filter and a frequency domain receiving filter forthe second radio; and apply the time domain receiving filter and thefrequency domain receiving filter in response to determining that thefirst radio is transmitting the first signals in the frequency bandwhile the second radio is receiving the second signals in the frequencyband.
 6. A computer program product, comprising: a non-transitorycomputer-readable storage medium having computer readable program codeembodied therewith, wherein the computer readable program code isconfigured to: train, for an access point comprising a first radio and asecond radio, a receiving filter for the second radio to mitigateinterference between the first radio and the second radio on a secondchannel of a frequency band including at least a first channel and thesecond channel, wherein the first radio is disposed in a body of theaccess point and configured to transmit first signals in the frequencyband, and wherein the second radio is disposed in the body of the accesspoint and configured to receive second signals in the frequency band,wherein training the receiving filter comprises: transmitting a requestto transmitting devices, other than the first radio, to not transmitsignals in the frequency band during a reserved time period, wherein therequest comprises a Clear-to-Send-to-self frame; transmitting a trainingsignal, by the first radio on the first channel in the frequency bandduring the reserved time period, and adjusting the receiving filter todecrease reception of testing signals received by the second radio onthe second channel during the reserved time period in which the firstradio is transmitting the training signal, wherein the testing signalsare portions of the training signal that leak into the second channelduring the reserved time period; apply the receiving filter to thesecond signals in response to determining that the first radio iscurrently transmitting the first signals while the second radio isreceiving the second signals; and remove the receiving filter from thesecond signals in response to determining that the first radio is notcurrently transmitting the first signals.
 7. The computer programproduct of claim 6, wherein the frequency band comprises a 5 GHz Wi-Fifrequency band.
 8. The computer program product of claim 6, wherein thefirst radio is configured to transmit the first signals in the frequencyband through a first plurality of antennas disposed on the body, andwherein the second radio is configured to receive the second signals inthe frequency band through a second plurality of antennas disposed onthe body.
 9. The computer program product of claim 6, wherein the firstradio is configured to transmit the first signals on the first channelin the frequency band and wherein the second radio is configured toreceive the second signals on the second channel in the frequency band.10. The computer program product of claim 6, wherein the computerreadable program code is further configured to: determine a time domainreceiving filter and a frequency domain receiving filter for the secondradio; and apply the time domain receiving filter and the frequencydomain receiving filter in response to determining that the first radiois transmitting the first signals in the frequency band while the secondradio is receiving the second signals in the frequency band.
 11. Amethod, comprising: transmitting first signals in a frequency band thatincludes at least a first channel and a second channel different fromthe first channel by a first radio disposed in a body of an accesspoint; receiving second signals in the frequency band by a second radiodisposed in the body of the access point; training a receiving filterfor the second radio to mitigate interference between the first radioand the second radio on the second channel of the frequency band,wherein training the receiving filter comprises: transmitting a requestto transmitting devices, other than the first radio, to not transmitsignals in the frequency band during a reserved time period, wherein therequest comprises a Clear-to-Send-to-self frame; transmitting a trainingsignal, by the first radio on the first channel in the frequency bandduring the reserved time period, and adjusting the receiving filter todecrease reception of testing signals received by the second radio onthe second channel during the reserved time period in which the firstradio is transmitting the training signal, wherein the testing signalsare portions of the training signal that leak into the second channelduring the reserved time period; applying the receiving filter to thesecond signals in response to determining that the first radio iscurrently transmitting the first signals while the second radio isreceiving the second signals; and removing the receiving filter from thesecond signals in response to determining that the first radio is notcurrently transmitting the first signals.
 12. The method of claim 11,further comprising: transmitting the first signals in the frequency bandthrough a first plurality of antennas disposed on the body; andreceiving the second signals in the frequency band through a secondplurality of antennas disposed on the body.
 13. The method of claim 11,further comprising: transmitting the first signals on the first channelin the frequency band; and receiving the second signals on the secondchannel in the frequency band.
 14. The method of claim 11, furthercomprising: determining a time domain receiving filter and a frequencydomain receiving filter for the second radio; and applying the timedomain receiving filter and the frequency domain receiving filter inresponse to determining that the first radio is transmitting signals inthe frequency band while the second radio is receiving signals in thefrequency band.